Both age and species diversity are critical in protecting a community’s urban forest, Miller said.

“Problems, such as insects, disease, and weather-related issues are usually specific to a plant species or family,” Miller said. “If communities plant less than 10 percent of a tree species and 20 percent of a tree family, then the chances of catastrophic failure due to insect or disease infestations is greatly reduced.”

In the past, Dutch elm disease wiped out boulevard trees in some communities and currently the emerald ash borer is causing a problem. A disease related to maples could be catastrophic to most communities in this part of the country where maple is the dominant tree.

Many trees are of similar age, Miller said, and that can pose a different problem. Creating different age classes will avoid a situation where the majority of the trees reach old age and die at about the same time.